Interleukin-2 (IL-2), originally identified as a T-cell growth factor, is possibly the most studied cytokine, yet newer physiologic functions continue to be ascribed to it. In addition to its pivotal role in regulating tolerance, IL-2 signals are criticl for generating functional CD8 T cell memory to pathogens. We recently demonstrated a critical role of IL-2 in balancing effector and memory responses during acute viral infections. Moreover, contrary to common belief that effector CD8 T cells do not produce IL-2, in a surprise finding we discovered that memory-fated effector cells produce large amounts of IL-2 and retain this ability throughout effector and memory stages. Indeed, robust IL-2 production is a hallmark property of long-lived polyfunctional memory cells and is required for robust secondary expansion. In one of the most stringent tests for physiological relevance of autocrine IL-2 signals, we adoptively transferred equal numbers of wild-type and IL-2-/- antigen-specific CD8 T cells into wild-type recipients, and followed memory differentiation within the same host in response to acute infection. Excitingly, we found that despite being in an environment where all other hematopoietic and nonhematopoietic cells and half of the antigen-specific CD8 T cells were capable of producing their own IL-2, IL-2-/- memory CD8 T cells were compromised in mounting robust secondary expansion upon rechallenge. This focused R03 proposal seeks to further extend these promising preliminary observations towards gaining insight into when autocrine IL-2 signals regulate cardinal CD8 T cell memory property of secondary expansion, and how paracrine IL-2 sources participate. Using novel conditional IL-2 knockout mice, we will extinguish autocrine or paracrine IL-2 signals either during primary CD8 T cell expansion (when memory properties are imprinted), or during secondary expansion phases (when memory cells undergo rapid proliferative burst and effector differentiation) to explore the timing of action of L-2 signals. We will employ phenotypic, functional and transcriptional profiling studies to gain molecular insight into how IL-2 signals regulate CTL immunity. For development of efficacious vaccines and immunization strategies that induce protective, long-lived CD8 T cell immunological memory, it is important to know why memory CD8 T cells produce their own IL-2, when and how this regulates memory properties and whether IL-2 signals can be provided in trans for optimal memory responses. Moreover, these studies lay the developmental groundwork for a future R01 proposal - narrowing down the time frame of action of autocrine IL-2 signals and a clear identification of the broad cellular and molecular networks involved will help us appropriately focus our future mechanistic investigations of autocrine IL-2 regulated gene regulatory networks under the auspice of a R01 funding mechanism.